Gravel packing of wells involves placing sized gravel or sand within a wellbore external to a screen. The gravel pack sand and screen slots are sized to prevent formation sand migration into the wellbore. Gravel packing is critical to continued oil production from subterranean formations consisting of loosely consolidated or unconsolidated sand. Without an effective gravel pack, the produced sand from the formation will erode equipment and fill tanks and vessels, thus causing the need for shutdowns to clean or replace equipment. Sand can also fill the wellbore and interfere with effective oil lifting. Additionally, displaced sand can leave cavities around the wellbore and thus remove lateral support from well tubulars. Removal of lateral support may result in damage to well tubulars.
The screen is typically hung from a packer and extends downward into a borehole. The borehole can be either cased or uncased. An annulus between the screen and the casing is typically sealed on top and bottom by packers.
Placement of sand around the screen is accomplished by circulating a slurry of sand suspended in a carrier fluid from the surface through a drill string to a crossover tool immediately above the screen. The crossover tool directs the slurry from the drill string to the annulus surrounding the screen. As the slurry travels down the annulus around the screen, some carrier fluid may enter the formation, leaving sand deposited at the perimeter of the wellbore or perforations. The remaining carrier fluid will pass through the screen. The screen has openings that are sized to retain the sand outside of the screen. Sand will therefore be filtered from the slurry by the screen and remain in the annulus outside of the screen. A wash pipe is usually provided within the screen extending from the crossover tool to near the bottom of the screen. Returning carrier fluid, after passing through the screen, enters the washpipe at the lower end of the wash pipe, and travels up the wash pipe to the crossover tool. The crossover tool directs this returning carrier fluid to the annulus outside of the drill pipe, above the screen packers, and up to the surface.
After the annulus outside of the screen is filled with sand, the crossover tool is disconnected from the screen, and removed from the wellbore. The screen is left suspended from packers, surrounded by gravel pack sand. A production tubing is then run to the inside of the screen, along with artificial lift means, if required. A crossover tool is provided to the gravel pack assembly as it is installed in the well. The crossover tool is a removable mechanism attached to the gravel pack packer which directs fluid flow while placing the gravel pack slurry. Slurry is pumped into the drill pipe down the wellbore. The slurry is diverted into the screen annulus via the crossover tool once the slurry reaches the gravel pack packer. As this occurs some may enter the formation and some may enter the screen. Since sand cannot pass through the screen, it is deposited in the screen annulus. Carrier fluid that enters the screen may return up a washpipe which is installed inside the screen. Fluids returning up the washpipe are diverted into the drillpipe/casing annulus above the gravel pack packer via the crossover tool to return to surface.
To be effective, the gravel pack must comprise densely packed sand without voids or cavities in the sand. If portions of the annulus around the screen are not packed completely with sand, formation fluids containing formation sand will quickly erode the screen, leading to a gravel pack failure. Further, if the gravel pack initially is not densely packed, subsequent compaction caused by, for example, flow of the formation fluids, can result in voids and cavities within the gravel pack.
Known methods to increase the density of gravel packs include pulsing the flow of the return fluid as disclosed in U.S. Pat. No. 3,830,294. Pulsing of the fluid flow is helpful in increasing the density of the gravel pack, but merely pulsing the flow imparts a limited amount of energy into the gravel pack, and can have deleterious effects, such as fracturing the formation. Other methods to pulse flows of drilling fluids have been developed for the purpose of transmitting information to the wellhead. These are described in, for example, U.S. Pat. Nos. 4,291,395, 4,323,991, 4,775,016 and 5,009,272. Like '294, these methods may not impart a significant amount of energy into a gravel pack.
A method to vibrate a drillstring and gravel pack apparatus by imparting a sonic frequency vibration which may be a resonant frequency at the wellhead is disclosed in U.S. Pat. Nos. 4,599,031 and 4,665,980. This method would be useful if the drill pipe does not have significant contact with the wellbore or casing above the gravel pack. Unfortunately, this is rarely the case. Wellbores inherently drill in a corkscrew configuration. A drill pipe is therefore in frequent if not almost continuous contact with the wellbore walls or casing. Vibrations imparted at the wellhead can therefore be significantly dampened, and vibrations of only a small magnitude may be present at the gravel packing apparatus. It would be preferable to impart vibrations at the gravel packing apparatus directly to minimize dampening of the vibrations.
U.S. Pat. No. 3,113,621 discloses the use of known vibration imparting tools to well liners to add gravel to a wellbore through the liner by vibrating larger sand particles through the liner than can return into the liner without vibration. The method to impart the vibration requires using known electrically driven or hydraulically driven vibrators. Thus, a 10 to 100 horsepower motor, along with a power source, must be inserted into the wellbore. It would be preferably to impart such vibration with a more simple, less expensive, and more compact source of vibration.
Bluff objects are known to shed vortices at rates which can be proportional to the flow rate of fluid passing the bluff object. This phenomena is utilized in flow rate measuring devices disclosed in, for example, U.S. Pat. Nos. 3,535,927, 3,927,566 and 4,026,150. Patent '566 further discloses the use of vortex shedding of one bluff object to move a second bluff object up and down. The second bluff object is located immediately downstream of the first bluff object. The frequency of the up and down movement of the second bluff object is, according to Birkoff's Theory, proportional to the rate of flow of the fluid past the bluff objects. Although the second bluff object could impact the walls of the flow conduit, such impacting would render the device unreliable as a flow measurement device and thereby defeat the intended purpose of the invention.
In one embodiment of the present invention, it is therefore an object to provide a method to impart vibration to a fluid flow conduit by a simple and reliable method. It is a further object to impart a vibration by a simple mechanism where the energy required for the vibration is derived from the energy of the flowing fluid.
In another embodiment, it is an object of the present invention to provide a simple method to produce a fine dispersion of fluid in another immiscible fluid.